We present a method to identify and quantify methane using a hydrophobic ionic liquid (IL)–electrified metal electrode interface by electrochemical impedance spectroscopy. We investigated the mechanisms of the responses of the IL-electrified electrode interface to the exposure of methane and other interfering gases (H2, C6H12, SO2, NO, NO2, CO2, O2, H2O). Our results show that at low frequency the IL-electrified electrode interface shows a predominantly capacitive response. The IL-electrode double layer (EDL) was found to be the primary response layer while the transition zone and bulk region of the IL-electrode interface contribute little to the overall signal change. For recognition and quantification of methane using the Langmuir adsorption model and measurement of differential capacitance change, an optimum EDL interface structure was found to form at a specific DC bias potential. The cumulative results shown in this work suggest that an ideal IL-electrode interface can be formed by varying IL structure and applied DC bias electrode potential for a specific analyte and that the semi-ordered structure of IL-electrified interface can act as a recognition element for the sensitive and selective adsorption and detection of gaseous molecules.
Following the 1980s PC revolution and the 1990s internet revolutions, recent decades, have experienced a revolution in sensor research which promises to have a significant impact on a broad range of applications including national security, health care, environment, energy, industry and food safety, and manufacturing.This articlehighlights a notable contributionto the sensor fields using ionic liquids (IL). Ionic liquids composed of sterically mismatched ions that hinder crystal formation were widely used as nonvolatile solvent. However, their crystal properties as molten salt have not been fully recognized. Different from other liquids, the innermost layer structure of ionic liquid/electrode is compact and ordered which can be utilized for selective gas sensing. We have shown that by selection of IL [C4mpy][NTf2] which has double layer structure favoring small gas molecule adsorption and by selecting the bias DC voltage at -0.3V based on the capacitance-potential curve maxima, highly sensitive and selective impedance CH4 sensor was demonstrated. The selectivity comes from the unique highly ordered arrangement of ions in the innermost layer of IL-electrode interface which is potential dependent. The degree of ordered structure at IL-electrode interface can be tuned by the applied bias potential (e.g. -0.3V for CH4) on the electrode and by the unique molecular structure of the IL ions. The high viscosity of ILs that is usually considered a limitation to practical electrochemical applications due to slow rate of mass transport is an advantage in capacitance measurement due to more ordered and concentrated double layer. The simplicity of the demonstrated detection principles allows for easy integration with engineering advancements such as portable electronics, networked sensing and next-generation monolithic implementation of autonomous sensors with the performance, cost, power, and operational lifetime characteristics to suit a broad range of applications.